1. Field of the Invention
The present invention relates to a method for manufacturing a hydrogen-occlusion-alloy electrode and more particularly to a method for manufacturing a nickel-hydrogen storage battery using the electrode as a negative electrode thereof to achieve reduced pressure inside the battery at the time of charging, a longer cycle life, and improved rapid discharging characteristic.
2. Prior Art
As the variety of electric and electronic equipment grows smaller in size, lighter in weight and are made cordless, the batteries used for their power supplies must be made increasingly smaller and lighter but with higher capacity.
A nickel-hydrogen storage battery is recently attracting attention as a high-capacity battery which responds to such demand.
The nickel-hydrogen storage battery operates using hydrogen as a negative electrode active material thereof; it comprises a negative electrode comprising a hydrogen-occlusion-alloy, which permits reversible occlusion and release of hydrogen and which is supported on a current collector, and a positive electrode comprising a nickel hydroxide, which normally functions as a positive electrode active material, the nickel hydroxide being supported on the current collector, and the negative and positive electrodes being disposed in an alkali electrolyte via a liquid retentive separator.
The hydrogen-occlusion-alloy electrode used for the battery is generally manufactured as set forth below.
First, a hydrogen-occlusion-alloy having a specified composition is prepared using an arc melting furnace, for example. An ingot thus obtained is mechanically crushed into powder of a desired particle size by using a crusher such as a ball mill and a hammer mill.
Then, the powder is directly mixed with a binding agent such as polytetrafluoroethylene or vinylidene fluoride and the mixture is molded into a sheet, thus producing the hydrogen-occlusion-alloy electrode.
There is another method for manufacturing the hydrogen-occlusion-alloy electrode. Unlike the method described above, in this method, a slurry is prepared by dispersing the aforesaid alloy powder in a solution which consists of a thickener such as methyl cellulose, carboxymethyl cellulose, and ethylene oxide dissolved in ion-exchange water or distilled water. Then a current collector such as a perforated nickel sheet is immersed in the slurry then drawn up so that the slurry is attached to the surface of the current collector. Next, the attached slurry is dried and the whole sheet with the dried slurry is rolled to provide the surface of the current collector with a hydrogen-occlusion-alloy powder layer of a desired thickness, thus producing the hydrogen-occlusion-alloy electrode.
In the nickel-hydrogen storage battery, a potential level at which charging reaction of the hydrogen-occlusion-alloy takes place is a value which is close to an electrolytic potential of the water constituting the alkali electrolyte. For this reason, during a charging process, water electrolysis may partly lead to generation of hydrogen. Hence, at the end of the charging process, the pressure inside the battery rises because the gas pressure of the hydrogen gas, which has been generated from the water electrolysis, is added. Such a rise in the internal pressure can be controlled to a certain extent by increasing the capacity of the negative electrode. This, however, results in an increased size of the battery, and therefore, it is not desirable in the light of the demand for compact batteries with a higher energy density.
Like other storage batteries such as a nickel-cadmium storage battery, the capacity of the nickel-hydrogen storage battery gradually decreases as charging and discharging is repeated until the life of the battery finally expires.
In the case of the nickel-cadmium storage battery, the life of the battery is determined to have expired when the capacity thereof has reached 60% or less of the rating. The battery is required to be capable of repeating at least 500 charge and discharge cycles before the life thereof expires.
In the case of the nickel-hydrogen storage battery, which is conventionally known, the number of charge and discharge cycles before the capacity of the battery drops to approximately 80% of the rating thereof ranges from 300 to 350 and the number of the charge and discharge cycles before the capacity reduces to approximately 60% of the rating ranges from 350 to 400. In other words, the capacity of the battery reduces to approximately 80% and 60%, respectively, when the charge and discharge cycle is repeated for the numbers of times mentioned above. It further means that the service life of the nickel-hydrogen storage battery expires after 300 to 400 charge and discharge cycles.
Hence, there is demand for developing a nickel-hydrogen storage battery which has a longer service life.